Method of forming polyvinyl acetal resin sheets



Patented Feb. 21, v1939 METHOD or PATENT OFFICE- roams ronxvmn. sonar.nssnv snssrs Gustavus J. Esseleu, swampscothand Elmer B.

D r y, Sin-infi l poration oi Delaware Np Drawing. Application November28,

Serial No. 242,814

The present application is a continuation in part of our priorapplication Serial No. 79,717, filed May 14, 1936.

Our present invention relates to the formation of polyvinyl acetal resininto a continuous sheet or web from which pieces may be cut for use inthe arts and particularly in the manui'ac-. ture of laminated glass,such as is employed in motor vehicles. By polyvinyl acetal resin, werei'er to themateriais described in the patent to Morrison, Skirrow andBlaikie, No. 2,036,092, dated March 31, 1936, and in the French patentto Carbide 8: Carbon Chemical Corporation No. 793,175. To give to thismaterial the qualities which are desirable if itis to be used as aninner layer for laminated glass certain compatible plasticizers, forinstance, dibutyl, diethyl and dimethyl phthalate, oarbitol acetate,di-ethyleneglycol-dipropionate must be mixed with it. Heretofore, in themanufacture of sheet material of this kind whether by the cake processor the continuous process it has been thought necessary to make amixcontaining a substantial amount of volatile solvent as well as theother ingredients. The volatile solvent was thought to be necessary toconvert the mix into a suitable state for handling, but was the sourceof serious losses since all the solvent was removed during seasoning,the expense of solvent recovery being greater than the value of anysolvent which might be recovered. Not only was the solvent lost andwasted, but the labor necessary to incorporate it in the mix, andafterwards of seasoning the stock to remove the solvent, were whollywasted. The fire hazard was serious ow-. ing to the inflammablecharacter of the solvent and in certain cases of the stock itself.Difiiculty was also experienced as there was a tendency of the volatilesolvent to form bubbles during processing. Polyvinyl acetal resin is sotenacious of certain volatile liquids which may be mixed with it torender it soft enough for extruslon in the form of a web that thenecessary removal of these liquids during curing is a long, expensiveand diflicult process. An extremely long ribbon must be retained in thedrying apparatus so that there is a large'amount of stock in process andthe machine required is inconveniently, long and expensive. Furthermore,although the material is thermo-plastic and may be softened by heat, itis so sticky and tacky, particularly when proportions of plasticizer areused which are suilicient to meet the present day requirements of thesafety glass manufacturers ,that the stock tends toadhere tenaciously YMass, assignors to Moncanto Chemical Company, St. Louis,

llosacor- (GI. 18-57) Q to all surfaces with which it contacts, such asthe die lips, rollers or rods over which it must be led during curingand an unsatisfactory ribbon or web is produced. The liquids in the mixgreatly increase the tendency of the characteristically limp and rubberyribbon or web to stretch and makeit necessary to support it at frequentintervals and increase the difllculty of handling during seasoning. Websproduced from. this material not only are rubbery and tough, but whenstretched contract slowly for a long time. Altogether this has causedserious losses and expenses, as a result of putting intothe mix asomething which has to be removed to make the final product. ,7

Retention of liquids, even although very small I quantities remain, also'givesundesirable properties to the web, such as and lack d! stability.It reduces the strength of laminated glass at temperatures such as aremet with in the, tropics and tends to the formation of bubbles at thehigh temperatureoi the enamel balnng oven'to which the laminated glassmay be subjected later. 2 We have discovered that all these'losses'anddifliculties may be done away with if the polyvinyl acetal resin ismixed with compatible plas ticizers but without solvent liquids and isthen formed into a web by extrusion through a suitable die. By employingsuitable plasticizers in suitable quanfities, the solvent liquids whichwould otherwise be required may be omitted altogether and the webproduced by the extrusion die is not so sticky and tacky, particularlyif chilled immediately upon emerging from the die, that it cannot behandled with a reasonable degree of convenience over rollers. No curingis required because the web contains no solvent. Consequently, theexpense for driers or seasoning chambers, for factory space, and iorstock in process is greatly reduced, and the other dimculties areavoided or are lessened.

In practicing our invention, a satisfactory mix consists of 100' lbs. ofpolyvinyl acetal resin with lbs. oi diethylene-glycol-dipropionate. Thepolyvinyl acetal resin employed is made by 11!- drolizing a polyvinylacetate until it is about 90 to 91% hydrolized. The resulting compoundis then combined with formaldehyde to such an extent that the finalproduct contains about 8% of polyvinyl alcohol which has not reactedwith the formaldehyde. In other words, the final. product containsbetween and of the acetal compound the remainder being partly polyvinylalcohol and partly polyvinyl acetate. It 55 should be understood,however, that the resulting product is a highly polymerized substance inv which the acetal compound, the polyvinyl alcohol and the polyvinylacetate are all combined together in'large molecules and therefore theexact determination of the proportions of each is a matter ofdiiilculty. The above description of the material is given by way ofillustration and it is not intended to limit the invention therebyexcept as may be expressly stated in the claims. It will be understoodthat other polyvinyl acetal resins of varying characteristics such asthose described in the two patents mentioned above may also be employed.Another example of a suitable mixture is 100 lbs. of polyvinyl acetalresin or the kind already described together with lbs. ofdiethylene-glycol-dipropionate and a certain amount of some suitablediluent, for instance, 5 lbs. camphor or 1 lb. of glycol laurate. Thismixture produces a less tacb and sticky sheet. Webs or sheets producedfrom material of this character are rubbery and tough, and have theproperty of contracting slowly and for a long time if stretched.

The polyvinyl acetal resins and the plasticizers and diluent or diluentsif used, are mixed in a suitable mixer, for instance. a Werner andPfleiderer machine, at a temperature of about 220 I"., the mixingbeingcontinued for two hours more or less until a homogeneous mass.results. The mass then goes into the extruder, being forced between thedie lips of a suitable die and coming out in the form of a sheet of adesired thickness, the thickness being determined primarily by the widthof the space between the die lips, though, as is well understood bythose skilled in the art, the thickness is affected by other factorssuch, for instance, as the pressure of extrusion, the viscosity of themixture, the temperature of extrusion, and the rate at which it is takenaway from the die lips. In practice it is desirable to chill the sheetto room temperature or lower immediately on emerging from the orifice ofthe die to set and harden it. This chilling may be done artificially asby blasts of air applied to opposite sides of the sheet. By artificiallychilling is meant any procedure other than mere exposure to normal roomatmosphere and adapted to lower the temperature 01 the sheet morequickly than would be effective by exposure to normal temperature. Ifthis procedure is employed, it is useful to use blasts of air onopposite sides of the sheet, the blasts being strong enough to cause thesheet to flutter violently at the die. This fluttering appears to havean important beneficial eilect on the operation of the machine and webelieve that it tends to release the adhesion of the stock to the dielips.

By employing the hereindescribed method, we are able to form rapidlysheets of stock from polyvinyl acetal resin even though plasticizers areused of such a nature and in such quantities as to render the stockstrongly adherent to highly polished surfaces such as metal and glasswhen heated to the temperatures and under the pressures now ordinarilyemployed in making laminated glass. We find that byusing this process weare able to form continuous sheets or webs of this tacky material atcommercially practical speeds.

Bince no volatile solvent is employed, the consistency or viscosity ofthe mix is more constant and is subject to better control. Therefore,difilculties in a continuous process of manufacture multing fl'omchanges in consistency are much aisaooa less troublesome than with a mixwhich includes volatile solvent some of which is lost with consequentvariation in consistency during the process of manufacture.

Another example which produces satisfactory results is as follows:

A polyvinyl acetal resin is made by combining a partially hydrolysedpolyvinyl acetate with butyraldehyde under such conditions as to give aproduct containing approximately 18-20% uncombined polyvinyl alcohol,1-2% polyvinyl acetate and the balance substantially acetal. This ismixed with 43 parts of triethylene glycol dihexoate to each parts of thevinyl acetal resin. No volatile solvent is used.

The resin and plasticizer are mixed together in any convenient type ofmixer as, for example, a jacketed kneading machine and the mass isagitated until it takes on a sponge-like consistency. If this mixingoperation is carried on at a temperature of approximately F. theoperation takes about one hour for a batch of 200 or 300 lbs. of resin,The resin is put in the machine first and the plasticizer is then pouredin, being distributed as well as possible. Where two or moreplasticizers are used they are mixed in the required proportions beforebeing added to the resin.

After the mixture has assumed the uniform sponge-like consistency it istransferred to a Banbury mixer which is at a temperature ofapproximately 180" F. Within two or three minutes the batch begins todecrease in volume, loses its spongy quality and begins to weld into atough, translucent mass. Cold water is then circulated through thejacket of the Banbury mixer and mixing is continued for from twelve tofifteen minutes longer after which the entire mass will have becometough and rubbery. Even though cold water has been circulated throughthe jacket of the mixer the internal heat raises the temperature of the.mix to approximately 280 F. at

The mix is forced through a fine chrome steel r screen backed by a Monelmetal screen. Particles of dirt and foreign matter are removed by thesefilter media. The strainer plate which supports the filter media isheated to between 240 to 310 E, the lower temperature range being"preferred. The cylinders of the hydraulic strainers are heated tobetween and 220 F. about 180 F. being preferred.

From the outlet of the hydraulic strainer the filtered stock is fedunder pressure to an extrusion machine in which the extrusion slot isformed between a die blade and a die roll, this machine being of thetype shown in the application of Parkhurst and Derby, Serial No.142,352, filed May 13, 1937, in which the temperature of the body of themachine is maintained between 280 and 320' R, preferably about 300 R,and the die blade is maintained between 230 1". and 280 'F., about 2401''. being preferred; the die roll is maintained at 120 1''. to 150' It,about 130' I. being preferred; the take-off roll is maintained between160 F. and 200 F., about 180 F. being preferred. The temperature 'of thestock at the moment of sheetformation, i. e., when it passes under thedie blade, is difficult to ascertain and is not uniform throughout thethickness of the sheet since the surface in contact with the die roll ischilled, while the surface in contact with the die blade is stillrelatively hot. Before contacting with the die roll, its temperature isbetween 160 F. and 220 F., preferably about 200 F. For some formulationswhere the quantity of plasticizer is increased and the plasticisconsequently more heat sensitive, a downward revision of 10 to 20 F.,in temperature is allowed. Although the die roll is maintained at atemperature above that of the atmosphere, it has the eflect of chillingthe stock since its temperature is much below that of the stock itself.The pressure in the chamber behind the die orifice is maintained at 350to .400 lbs. per square inch. These operating conditions apply to aparticular extrusion unit operating on the mix described at a ribbonspeed of 10 to feet per minute and producing stock of 0.015 inchthickness.

i It should be recognized that there is an inter- I dependence betweentemperatures, pressures and speeds and that they are a function of thesize of the units as well as of the properties of the plastic.

We claim:

1. The method of making a continuous web of polyvinyl acetal resin whichincludes mixing and kneading substantially volatile solvent free resinand a sufficient amount of substantially volatile solvent freeplasticizer to produce a material which when. sheeted is adherent tohigh y bolished surfaces under heat and pressure, until a homogeneousmixture is obtained, softening the mixture by heat in the absence of avolatile solvent and forcing the softened mixture through the: slot of adie to form it into a continuous w 2. The polyvinyl acetal resin whichincludes mixing and kneading substantially volatile solvent free resinand a sumcient amount of substantially volatile solvent free plasticizerto produce a material which when sheeted is adherent to highly D 1-method of making a continuous web of ished surfaces under heat andpressure, until a homogeneous mixture is obtained, softening the mixtureby heat in the absence of a volatile solvent and for'cingthe softenedmixture through the slot of a die to form it into a continuousselfsustaining web and then artificially chilling the web.

3. The method ofmaking a continuous web of polyvinyl acetal resin whichincludes mixing and kneading substantially 100 lbs. of substantiallyvolatile solvent free polyvinyl acetal resin and about lbs. by weight ofsubstantially volatile solvent free compatible plasticizer to produce amaterial which when sheeted is adherent to highly polished surfacesunder heat and pressure until.

solvent free plasticizer to produce a material which when sheeted isadherent to highly polished surfaces under heat and pressure, until'ahomogeneous mixture is obtained, softening thev mixture by heat at atemperature of about 200 F. in the absence of a volatile solvent andforcins the softened mixture through the slot of a die to 7 form it intoa continuous web and then artificially chilling the formed web.

5. The method of making a continuous web of polyvinyl acetal resin whichincludes mixing and kneading substantially volatile solvent free resinand of sufficient amount of substantially volatile.

solvent free plasticizer to produce a material which when sheeted isadherent to highly polished surfaces under heat and pressure, until ahomogeneous mixture is obtained, softening the mixture by heat intheabsence of a volatile solvent and forcing the ,softened mixture throughthe slot of a die to form it into a continuous selfsustaining web andchilling it artificially as it emerges from the die orifice.

ous'rsvus J. arm a. mass.

